1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to calibration of circuit breakers including a thermal trip assembly. The invention also relates to methods of thermally calibrating circuit interrupters.
2. Background Information
Electrical switching apparatus, such as circuit interrupters, include an operating mechanism and a trip device, such as a thermal trip assembly and/or a magnetic trip assembly, collectively, “the trip assembly.” The operating mechanism is coupled to a number of separable contacts that move between a first, open position and a second, closed position. The trip device is coupled to the separable contacts, e.g. via the operating mechanism, and cause the separable contacts to move from the second, closed position to the first, open position following an over-current event.
The trip device is automatically releasable to effect tripping operations and manually resettable following tripping operations. Examples of circuit breakers including trip mechanisms are disclosed in U.S. Pat. Nos. 5,805,038 and 6,838,961. Such circuit breakers, commonly referred to as “miniature circuit breakers,” have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured and tested. As such, the ease of test of such circuit breakers is of importance.
Circuit breakers of this type include, for example, a non-conductive housing assembly in which the other components are disposed. The separable contacts include a fixed contact attached to the housing assembly and a movable contact coupled to the operating mechanism. The operating mechanism includes a movable handle that extends outside of the housing assembly. Movement of the separable contacts is accomplished by the operating mechanism. The trip device includes a cradle and the previously mentioned trip assembly including a thermal trip assembly and/or magnetic trip assembly. The cradle is coupled to a spring and disposed between the trip device and the operating arm. The components further include a frame to which the other components are coupled.
The frame includes a generally planar body that is coupled to a sidewall in the housing assembly. The trip assembly is coupled to the frame. The cradle engages an opening on the trip assembly. The cradle is biased, e.g. by a spring, and if free to move will cause the operating mechanism to move the separable contacts from the second, closed position to the first, open position.
The thermal trip assembly includes a bimetal element (hereinafter, “bimental”). When exposed to a first predetermined over-current condition, the bimetal is heated and deforms. The deformation of the bimetal moves the opening on the trip assembly away from the cradle thereby releasing the cradle and opening the separable contacts, as described above. The magnetic trip assembly includes a magnetic yoke that is flexibly coupled to the bimetal. The opening on the trip assembly is disposed on the magnetic yoke. In the event of an instantaneous second predetermined over-current condition, wherein the second predetermined over-current condition is higher than the first predetermined over-current condition, the bimetal generates a magnetic field that causes the magnetic yoke to move toward the bimetal. When the magnetic yoke moves, the opening on the trip assembly moves away from the cradle thereby releasing the cradle and opening the separable contacts, as described above. Thus, the position of the trip assembly relative to the cradle, and more specifically the position of the opening on the trip assembly relative to the cradle, affects the trip conditions, especially the thermal trip condition. That is, generally, if the trip assembly is closer to the cradle, the bimetal must deflect a greater amount before the cradle is released.
As noted above, the trip assembly is coupled to the frame. Thus, movement, or more specifically a deformation, of the frame affects the position of the trip assembly relative to the cradle. As such, deforming the frame may be used to calibrate the trip assembly. A detailed explanation of calibrating a trip assembly by deforming a frame is set forth in U.S. Pat. Nos. 6,239,676 and 7,859,369. As noted therein, present frames include an opening with a calibration slot. That is, the frame is a planar body having an opening with a radial slot. In selected areas about the opening and calibration slot, the frame body is attenuated. Thus, a calibration tool was inserted into the calibration slot and twisted. This motion caused the frame body to deform at the attenuated portions, thereby deforming the frame body in a first direction and moving the trip assembly. To deform the frame body in the opposite direction, a two-pronged tool must be used so as to engage the frame body both from within the calibration slot as well as on the perimeter of the frame body.
As detailed in U.S. Pat. Nos. 6,239,676 and 7,859,369, the calibration tool must engage the frame body from a direction generally normal to the plane of the frame body. That is, the calibration tool must enter on a lateral side of the circuit breaker housing assembly. Following calibration, multiple circuit breakers may be coupled together. In this configuration, it is difficult, or in some instances impossible, to recheck the calibration after joining the circuit breakers because there little or no lateral access to the circuit breaker. That is, providing lateral access for the calibration tool is a disadvantage.
Further, as shown in FIG. 1, the frame body 1 must be shaped to define the opening 2 and the calibration slot 3 into which the calibration tool is inserted. That is, the frame body 1 must include a “loop” 4 that extends from medial portion of the frame body 1 to a location near the upper end 5 of one end of the frame body 1. Thus, the loop 4 defines the opening 2. That is, as sued herein, a “loop” is a portion of a support frame body that defines a part of a circular opening having a calibration slot and which is disposed generally opposite the calibration slot. Further, because the cradle biases the frame toward the bottom of the circuit breaker housing assembly, the frame may slowly deform over time. This deformation is known as “frame creep” and may move the trip assembly out of calibration.
Thus, the size, shape, configuration and orientation of the frame body, including lateral access for the calibration tool, is a disadvantage. There is, therefore, room for improvement in the frame as well as in methods of thermally calibrating circuit interrupters.